This invention relates to radio frequency pulse generating devices and, in particular, to high energy microwave pulse generators having a spark gap immersed in liquid.
It is known to produce bursts of microwave energy by switching power across a gap or gaps in the central conductor of a coaxial line or waveguide, as shown in U.S. Pat. No. 3,521,121 issued July 21, 1970 to J. M. Proud, JR., U.S. Pat. No. 3,484,619 issued Dec. 16, 1969 to J. M. Proud, Jr., U.S. Pat. No. 3,748,528 issued July 24, 1973 to H. Cronson and copending U.S. Patent application Ser. No. 661,677 entitled, "High Voltage Control Devices" to J. M. Proud, Jr., filed Feb. 26, 1976. These devices are capable of providing high energy pulse bursts of microwave power up to approximately 10 GHz. In the waveguide device disclosed in the patent to H. Cronson, the center conductor of the coaxial line is provided with one or more switching gaps along its length and/or the end post spaced from the interior wall of the waveguide. A radio frequency (RF) block or impedance is disposed around the post adjacent the first switching gap with the gaps so dimensioned that energy switched by the first gap can pass the block or impedance, but oscillations caused by the discharge at the second gap cannot pass the block. The spark gap functions to steepen the leading edge of the traveling wave. The prior art structures possess several shortcomings which the instant invention overcomes. These shortcomings include, but are not limited to, poor efficiency, since the capacitance of the RF block, to be effective, must be relatively large compared to the distributed capacity of the end post. The capacitance of the RF block stores most of the initial energy and, upon discharge, tends to react with the lumped value of inductance in the post, resulting in low frequency oscillations which typically are less than one-half the desired operating frequency. In the Cronson waveguide device, mentioned hereinbefore, such oscillations lie below the waveguide cut-off frequency and would not be observed. However, the existence of such oscillations have been observed by the applicant using coaxial resonators operating in the lowest TEM mode with no cut-off frequency. This problem cannot be overcome by reducing the capacitance of the RF block since this would permit the microwave oscillations to leak out of the resonant structure.
Furthermore, the Cronson device requires a spark gap of relatively small dimensions to provide rapid charging of the end post. This causes the small electrode surface of the gap to wear rapidly under sparking causing rapid deterioration of the fast switching properties of the closely spaced spark gap. A sliding short is also used to adjust the coupling between the waveguide and the end post. However, it does not enable tuning of the generator since the microwave frequency generated is determined primarily by the cross sectional dimensions of the waveguide.
It is also known, by those knowledgeable in the art, to use liquid or gaseous sparking mediums since their insulation and self-repairing properties may be superior to air. However, prior known liquids were found to decompose under repetitive sparking (arcing) conditions, thereby lowering the dielectric breakdown strength of the liquid. Insulating gases, to offer practical operating ranges, require pressurization in the spark area.
Therefore, it is an object of the present invention to overcome the shortcomings of presently known radio frequency generators.
It is a further object of the present invention to provide a highly efficient, high power radio frequency generator with a liquid insulating medium.
Another object of the present invention is to provide a liquid insulating medium which may be utilized in numerous spark gaps.
Still another object of the present invention is to provide a liquid sparking medium which does not substantially decompose under repetitive sparking conditions.
Yet another object of the present invention is to provide a sparking medium which also serves to remove heat from the sparking region.
A still further object of the present invention is to provide a novel microwave frequency generator which is small in size and relatively inexpensive to manufacture as compared to conventional RF microwave generators of equal output power.
The foregoing and other objects and advantages will appear from the description to follow. In the description, reference is made to the accompanying drawing which forms a part thereof, and in which is shown by way of illustration, a specific embodiment for practicing the invention. This embodiment will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims.